This invention relates to a system control unit of a data processing system, and more particularly, to a system control unit having multiple command level conditional rotational servicing priority hierarchy.
System control units of present systems have included a port priority based on port group, and as a result, a blocked port may be bypassed. The present invention incorporates command types having a predetermined priority level. Port servicing of the preferred embodiment of the present invention is based upon two levels of commands, high and low, the high level commands having priority over low level commands. Each level is independent of the other. The priority within the level is rotated upon servicing a port, provided that a higher priority port within the level does not have a service request pending which is blocked due to some necessary portion of the system being unavailable. An individual port user may change its command level within the established systems requirements if it is not selected in a predetermined select interval.